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NANO REVIEW Open Access
Properties of nanocones formed on a surface of
semiconductors by laser radiation: quantum
confinement effect of electrons, phonons, and
excitons
Artur Medvid
*
, Pavels Onufrijevs and Alexander Mychko
Abstract
On the basis of the analysis of experimental results, a two-stage mechanism of nanocones formation on the
irradiated surface of semiconductors by Nd:YAG laser is proposed for elementary semiconductors and solid
solutions, such as Si, Ge, SiGe, and CdZnTe. Properties observed are explained in the frame of quantum
confinement effect. The first stage of the mechanism is characterized by the formation of a thin strained top layer,
due to redistribution of point defects in temperature-gradient field induced by laser radiation. The second stage is
characterized by mechanical plastic deformation of the stained top layer leading to arising of nanocones, due to
selective laser absorption of the top layer. The nanocones formed on the irradiated surface of semiconductors by
Nd:YAG laser possessing the properties of 1D graded bandgap have been found for Si, Ge, and SiGe as well,
however QD structure in CdTe was observed. The model is confirmed by “blue shift” of bands in
photoluminescence spectrum, “red shift ” of longitudinal optical line in Raman back scattering spectrum of Ge
crystal, appearance of Ge phase in SiGe solid solution after irradiation by the laser at intensity 20 MW/cm
2
, and
non-monotonous dependence of Si crystal micro-hardness as function of the laser intensity.
1. Introduction
Many experimental and theoretical investigations exist on
heterostructures of self-assembled nanocones, e.g., Ge/Si
[1], InAs/GaAs [2]. Usually nanocones are considered as
quantum dots (QDs)–QD quantum system, with a condi-
tion ratio diameter/height of nanocones is equal 1. If solid
angle a at top is > 60°, then the nanocone transforms into
a quantum well (QW)–2D quantum system, due to large

diameter of nanocones in comparison with the height and
quantization of energy of particles (e.g., excitons) takes
place only in vertical direction [2]. The decrease of nano-
cones’ solid angle a < 60° leads to fundamental changes of
its prop erties. QD transforms into a quantum wire
(QWi)–1D quantum system with gradually decreasing dia-
meter from the base till the tip of the cone. The last one is
a unique system which has wide technical applications, for
example, 1D-graded bandgap structure in elementary
semiconduct or [3]. It is po ssible to form these two types
of quantum systems by laser radiation (LR).
Photo-thermo-deformation model [4] has been pro-
posed for explaining self-assembly of nanostructures on a
surface of a semiconductor by LR. According to this
model, conversion of light into heat and lateral deforma-
tion of the crystalline lattice of a semiconductor takes
place due to inhomogeneous absorption of light, leading
to formation of periodical structure on the surface due to
redistribution of point defects (interstitials and vacancies).
Nanostructures, such as, QD, QWi, and QW, are
formed in semiconductors by widely used methods, e.g.,
molecular beam epitaxy (MBE) [5], ion implantation [6],
chemical vapor deposition [7], laser ablation [8]. By these
methods, nanostructures mostly grow in random man-
ner, and parameters of such materials are not controlled,
it is the so-called self-assembly manner [9].
In this article, possibilities to control parameters of
nanocones, such as height an d distribution, on the sur-
face of a semiconductor by the Nd:YAG laser intensity,
* Correspondence:

Research Laboratory of Semiconductor Physics, Institute of Technical Physics,
Riga Technical University, 14/24 Azenes Str., Riga, LV-1048, Latvia
Medvid et al. Nanoscale Research Letters 2011, 6:582
/>© 2011 Medvid et al; licensee Springer. This is an Open Ac cess article distributed under the ter ms of the Creative Commons Attribution
License ( ), which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
wavelength, and pulse duration have been proposed.
Considering quantization of quasi-particles (e.g., exci-
tons, phonons, etc.) in nanocone is a special case, since
diameter of nanocone is a monotonous function of
height, leading to gradual change of bandgap. Graded
bandga p structure has an effect on properties of particles
and quasi-particles, such as mobility and intrinsic con-
centration of electrons a nd holes, energy of excitons,
phonons, and plasmons. Therefore, study of nanocones’
formation mechanism and nanocones’ properties is an
important task for future nanoelectronics and optoelec-
tronics industry.
2. Materials and methods
Ge (100) i-type single crystal samples with sizes 10.0 ×
5.0 × 5.0 mm
3
and resistivity r =45Ωcm were used in
experiments. The samples were polished mechanically
and etched in CP-4A (mi xture of 16% of HF; 64% of
HNO
3
and 20% of CH
3
COOH) solution to ensure mini-

mal surface recombination velocity S
min
=100cm/son
all the surfaces. Commercial p-andn-type Si(100), (111)
single crystals were investigated in the experiments. Solid
solution of SiGe, containing 30% of Ge atoms (Si
0.7
Ge
0.3
),
grown by MBE on top of a 150-nm thick Si buffer layer
on Si was studied in the experiments. High-purity solid
solution of CdZnTe, c ontaining 10% of Zn atoms
(Cd
0.9
Zn
0.1
Te), grown by high-pressure vertical zone
melting method was used in the experiments as well. The
grown crystals were cut into 10.0 × 10.0 × 1.0 mm
3
wafers. SiO
2
protective layer on the irradiated surface of
the samples was applied in the experiments with Si and
Cd
0.9
Zn
0.1
Te for preventing oxidation of Si nanocones

and evaporation of Cd atoms from Cd
0.9
Zn
0.1
Te surface.
Radiation by fundamental frequency of a pulsed Nd:
YAG laser for Ge single crystals and Si
0.7
Ge
0.3
solid solu-
tion with following parameters was used: wavelength l
1
=
1064 nm, pulse dura tion τ = 15 ns, pulse repetition rate
12.5 Hz, power P = 1.0 MW. For Si and Cd
0.9
Zn
0.1
Te sin-
gle crystals, the second harmonic of the laser with l
2
=
532 nm and τ = 10 ns was applied. Laser beam to the irra-
diated surface of the samples was directed normally. The
spot of laser beam of 3 mm diameter was scanned over
the sample surface using a two-coordinate m anipulator
with 20 μm step. All experiments of nanocones’ formation
were performed in ambient atmosphere at pressure of 1
atm, T = 20°C, and 60% humidity.

The surface morphology by atomic force microscope
(AFM) was studied. Optical properties of non-irradiated
and irradiated samples by photoluminescence (PL) and
back scattering Raman methods were investigated. For
PL, the 488-nm line of a He-Cd laser and for Raman
back scattering an Ar
+
laser with l = 514.5 nm were
used. Measurement of the PL spectra for Si, Ge, and SiGe
solid solution at room temperature was performed, but
for solid solution of CdZnTe–at 5°K. Detailed description
of these experiments is published in the following arti-
cles: for elementary semiconductors Ge [10] and Si [3]
and solid solutions Si
0.7
Ge
0.3
[11] and Cd
0.9
Zn
0.1
Te [12].
Microhardness test was performed using a microhardness
tester PMT-3 (manufactured by LOMO in USSR) using
indentation method with original self-adjusting loading
device, allowing to carry out precision microhardness
measurements at very small test loading. The indenter
was used a Vicker’s diamo nd pyramid and relaxation
time was 15 s. Each point in the figures corresponds to
20 measurements of processed statistically.

3. Results and discussion
The mechanism of nanocones’ formationontheirra-
diated surface of Si
0.7
Ge
0.3
solid solutions is character-
ized by two stages–laser redistribution of atoms (LRA)
and selective laser annealing (SLA) [13].
The first stage, LRA, i s characterized by formation of
heterostructures such as Ge/Si due to drift of Ge atoms
toward the irradiated surface of the sample in the gradient
of temperature, the so-called thermogradient effect (TGE)
[14]. This process is characterized by positive feedback:
after every laser pulse, the gradient of temperature
incr eases due to the increas e of Ge atoms’ concentration
at the irradiated surface. New Ge phase is formed at the
end of the process. Ge atoms are lo calized at the surface
of Si like a thin film. As a result, LRA stage gradually tran-
sits to SLA stage.
The second stage, SLA, is characterized by formation of
nanocones on the irradiated surface of a semiconductor
by selective laser heating of the top layer with following
mechanical plastic deformation of the layer as a result of
relaxation of the mechanical compressive stress arising
between these layers due to mismatch of their crystal lat-
tices and selective laser heating. SLA occurs due to
higher absorption of the LR by the top layer than the
buried layer.
A similar two-stage’s mechanism can be used for

nanocones’ formation by laser beam on ternary solid
solution Cd
0.9
Zn
0.1
Te. Irradiation of the Cd
0.9
Zn
0.1
Te
solid solution by the laser leads to the drift of Cd atoms
toward irradiated surface and of Zn atoms–in the bulk
of the semiconductor due to TGE [14]. As a result, for-
mation of CdTe/Cd
1-x1
Zn
x1
Te heterostruct ure, where x
1
> 0.1, takes place. Decrease of Zn atoms’ concentration
in the top layer with intensity of LR, according to the
proposed model, leads to the “red shift” of the exciton
bandsinPLspectra,aswasshownin[12],butincrease
of the Zn atoms’ concentration in buried CdZnTe layer
manifests in “blue shift” of the PL spectrum, as shown
in Figure 1 on the left side. The se effects do not com-
pensate each other since they take place in different
layers. Of course, it is possible to observe both PL
Medvid et al. Nanoscale Research Letters 2011, 6:582
/>Page 2 of 6

spectra simultaneously at intermedia te situation. Exactly
such situation is observed in PL spectrum, in Figure 1,
after destruction of the CdTe top layer and formation of
nanocones on the irradiated surface of the sample.
Relaxation of the mechanical compressive stress in
CdTe layer comes to an expression as self-assembly of
nanocones on the irradiated surface of the structure like
Ge nanocones in SiGe solid solution. Calculation of the
mechanical compressive stress in CdTe top layer using
the maximum of the “blue shift” of excitons bound to
shallow neutral acceptors (A
0
X) exciton band from Fig-
ure 1 and dE
g
/dP =10eV/Pa[10],whereE
g
and P are
bandgap of CdTe crystal and mechanical stress, respec-
tively, gives P =4.62×10
5
Pa. This value corresponds
to the ultimate strength limit of CdTe [11]. Calculation
of QD diameter using the formula from [12] and the
“blue shift” of A
0
XQCinPLspectrumon0.27eVgive
diameter of the QDs up to 10.0 nm. These data corre-
spond to the s ize of nanocones (height and diameter of
the bottom of the cones are abou t 10 nm with an error

of ± 1 nm) measured using 3D image of AFM. An evi-
dence of presence of the e xciton quantum confinement
in nanocones is the decrease of longitudinal optical
(LO) phonon energy by 0.7 meV in PL spectrum (as can
be seen from Figure 1, positions of A
0
X-LO and
A
0
XQC-LO zero phonon bands), that is the so-called
phonon quantum confinement effect [13]. Our calcula-
tion on Zn atom ’s distribution depending on intensity of
LR using the thermo-diffusion equation has shown that
the process of CdTe/Cd
1-x1
Zn
x1
Te heterostructure for-
mation is characterized by gradual increase of Zn atom’s
concentration in the buried layer with intensity of LR
up to 8%. It means concentration of Zn atoms is 0.18.
The thickness of the CdTe layer after irradiation by the
laser with intensity of I = 12.0 MW/cm
2
becomes 10 nm.
Moreover, the stress is caused by both due to large lat-
tice mismatch between CdTe and Cd
1-x1
Zn
x1

Te layers
[15] and SLA stage. Relaxation of the mechanical com-
pressive stress in CdTe layer as a result of nanocones for-
mation on the irradiated surface of Cd
0.9
Zn
0.1
Te sample
similar to Stransky-Krastanov’ mode [16] takes place.
Appearance of a new exciton band at 1.872 eV in PL spec-
trum of Cd
0.9
Zn
0.1
Te sample at higher intensity of LR was
observed. Reconstruction of this band (see Figure 1 on the
right side) shows that it consists of three lines which look
like A
0
X, D
0
X (excitons bound to shallow neutral donors)
and A
0
X-LO (phonon replica of excitons bound to shallow
neutral acceptors) lines in the non-irradiated PL spectrum
of the structure. Therefore, we connect both the new band
appearance in PL spectrum and the nanocones’ formation
Figure 1 PL spectra of the Cd
0.9

Zn
0.1
Te measured at temperature 5 K: curve 1, non-irradiated; curve 2, irradiated by the laser at I =
12.0 MW/cm
2
.
Medvid et al. Nanoscale Research Letters 2011, 6:582
/>Page 3 of 6
on the irradiated surface of the semiconductor with exci-
ton quantum confinement in nanocones and denote them
as A
0
XQC and D
0
XQC lines.
In the case of the elementary semiconductors, at the
first stage of the process, a thin top layer with mechani-
cal compressive stress due to separation and redistribu-
tion of interstitials and vacancies in gradient
temperature field [14] on the irradiated surface of the
semiconductors is formed. As a result, interstitials are
concentrated at the irradiated surface of semiconductor,
forming the top layer. Vacancies are concentrated under
the top layer forming a buried layer with mechanical
tension due to the absence of atoms. Sometimes vacan-
cies form nanocavities [17]. At the second stage of the
process, nanocones are formed on the irradia ted surface
of the semiconducto rs due to plastic deformation of the
top layer in the same way as in the previous case with
semiconductor solid solutions.

To approve two-stage mechanism of nanocones
formed on the semiconductor surface, we have proposed
several evidences:
1. Appearance of nanocones on the irradiated sur-
face of semico nductors and their height dependence
to the laser intensity has been found by measure-
ments of the irradiated surface morphology by AFM,
as shown in Figure 2.
2. The “blue shift” of the PL spectra and increase of
PL bands’ intensity of Si, Ge, and Si
0.7
Ge
0.3
crystals
with increase of the LR intensity due to quantum
confinement effect, as shown in Figure 3 for
Si
0.7
Ge
0.3
crystal, is the next evidence of the SLA
stage.
3. The presence of the first stage is appearance and
increase of intensity of LO phonon line with fre-
quency 300 cm
-1
in Raman back scattering spectrum
a
b c
Figure 2 AFM images of irradiated Si

0.7
Ge
0.3
solid solution. AFM images of Si
0.7
Ge
0.3
surfaces irradiated by the Nd:YAG laser at intensity (a)
2.0 MW/cm
2
; (b) 7.0 MW/cm
2
and (c) 20.0 MW/cm
2
.
0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2,0 2,1
1000
1500
2000
2500
I
ntens
i
ty, a. u.
Photon Ener
gy
, eV
I
1
=20.0 MW/cm

2
I
2
=7.0 MW/cm
2
I
3
=2.0 MW/cm
2
nonirradiated
Figure 3 PL spectra of Si
0.7
Ge
0.3
solid solution: nonirradiated and irradiated by Nd:YAG laser.
Medvid et al. Nanoscale Research Letters 2011, 6:582
/>Page 4 of 6
of Si
0.7
Ge
0.3
solid solution after irradiation by the
laser. A new Ge phase is observed on the irradiated
surface of Si
0.7
Ge
0.3
[18], as shown in Figure 4.
4. Non-monotonous dependence of Si crystal micro-
hardness as a function of the laser intensity. The

increase of microhardness with increasing LR inten-
sity is explained by formation of mechanically com-
pressed layer at the irradiated surface due to
increase of concentration of the interstitial atoms of
Si at the surface in temperature gradient field, which
is characteristic to the LRA stage. The decrease of
the microhardness is explained by formation of
nanocones as a resu lt of plastic deformation of the
mechanically stressed layer, which is characteristic to
the SLA stage, as shown in Figure 5.
5. The shift of PL spectrum of Cd
0.9
Zn
0.1
Te solid
solution at low intensity of the LR toward lower
energy of quantum –the “red shift” [12] –is the next
evidence of the first stage of thin CdTe layer forma-
tion. The shift of bands in PL spectrum of
Cd
0.9
Zn
0.1
Te solid solution at high intensity of the LR
toward higher energy of quantum–the “blue shift”
200 250 300 350 400 450 500 550
0
500
1000
1500

2000
bulkSi
Si-Si
Ge-Si
I
ntens
i
ty, a. u.
Raman shift
,
cm
-1
I
1
=20.0 MW/cm
2
I
2
=7.0 MW/cm
2
I
3
=2.0 MW/cm
2
Non irradiated
Ge-Ge
L
ex
= 5145
A

Figure 4 Back scattering Raman spectra of Si
0.7
Ge
0.3
solid solution: non-irradiated and after irradiation by the laser.
Figure 5 Microhardness of n-Si (111) wafer depending on laser intensity at load on indenter 20 g.
Medvid et al. Nanoscale Research Letters 2011, 6:582
/>Page 5 of 6
[12] and appearance of a new PL b and at higher
energy of quantum–exciton quantum confinement
effect, as shown in Figure 1, are evi dences of the sec-
ond stage of the mechanism.
5. Conclusions
1. For t he first time we have shown a possibility to
form 1D-graded bandgap structure in elementary
semiconductor. The graded bandgap is formed in
nanocones due to quantum confinement effect.
2. We have shown the possibility to control nano-
cones’ features by changing LR parameters, such as
intensity, wavelength, and pulse radiation duration.
3. The new PL band at 1.8718 eV is observed after
irradiation of Cd
0.9
Zn
0.1
Te solid solution by Nd:YAG
laser at intensity 12.0 MW/cm
2
. The origin of this
PL band we connect with exciton quantum confine-

ment effect in nanocones was formed on the irra-
diated surface of the semiconductor.
Abbreviations
A
0
X: excitons bound to shallow neutral acceptors; A
0
X-LO: longitudinal
optical (LO)-phonon replica of excitons bound to shallow neutral acceptors;
AFM: atomic force microscopy; D
0
X: excitons bound to shallow neutral
donors; LR: laser radiation; LRA: laser redistribution of atoms; MBE: molecular
beam epitaxy; PL: photoluminescence; QD: quantum dots; QWi: quantum
wires; QW: quantum well; SLA: selective laser annealing; TGE: thermogradient
effect.
Acknowledgements
The author gratefully acknowledges the financial support in part by the
European Regional Development Fund within the project “Sol-gel and laser
technologies for the development of nanostructures and barrier structures”
and by the European Project in the framework FR7-218000 “COCAE”.
Authors’ contributions
AM conceived the studies and coordinated the experiment. All of the
authors participated to the analysis of the data and wrote the article. AMy
carried out the sample preparation, the measurements for solid solutions of
CdZnTe. PO carried out the sample preparation, the measurements for
elementary semiconductors: Si, Ge and solid solution of SiGe. All the authors
read and approved the manuscript.
Competing interests
The authors declare that they have no competing interests.

Received: 17 June 2011 Accepted: 7 November 2011
Published: 7 November 2011
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Cite this article as: Medvid et al.: Properties of nanocones formed on a
surface of semiconductors by laser radiation: quantum confinement
effect of electrons, phonons, and excitons. Nanoscale Research Letters
2011 6:582.
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